1. Field of the Invention
This invention relates to a wear resistant drill bit for use in the formation of subterranean well bores.
2. Description of the Related Art
In order to maximize drilling efficiency it is important to minimize the down-time of a drilling rig which occurs when a bit requires replacement, and the frequency with which bits require replacement. Clearly, improving the ability of a drill bit to withstand the wear which occurs in use will reduce the frequency of bit replacement and so is advantageous. A number of techniques for improving the wear resistance of a drill bit are known. For example it is known to mount wear resistant components on the exterior of a steel bodied drill bit, as described in U.S. Pat. No. 6,092,613, or to apply a coating of a suitably wear resistant material to the drill bit. These techniques are used, primarily, with drill bits having bodies formed from cast or machined steel.
In another type of drill bit, the bit body is formed from one or more powders secured in a matrix by a binder material. Typically, with drill bit bodies of the matrix type, either a macrocrystalline tungsten carbide material is used in the matrix, or a crushed, cast tungsten carbide material is used. Both of these materials are thought to have advantages and disadvantages.
The use of the crushed, cast material results in the formation of matrix bit bodies of good erosion resistance but relatively low fatigue strength. Matrix bit bodies formed using the macrocrystalline material have a lower erosion resistance but improved fatigue strength. By way of example, the erosion resistance of a matrix bit body formed using the cast and crushed material is typically approximately five times that of a body formed using the macrocrystalline material, but has a fatigue strength of only about 40% of that of a body formed using the macrocrystalline material.
The reasons for these properties are thought to be that the crushed cast tungsten carbide takes the form of a mixture of WC and W2C whereas the macrocrystalline material consists only of WC. W2C is harder than WC and so the crushed cast material is more capable of withstanding abrasion or erosion than the macrocrystalline material. Further, the cast, crushed material is made up of particles of uneven shape with irregular and angular surfaces giving rise to a larger surface area than the macrocrystalline material, which is made up of crystals of a more regular, blocky form which have smooth surfaces. As a result, the chemical or metallurgical bond between the crushed, cast material and a binder material is somewhat stronger than that between the macrocrystalline material and the binder material. Mechanical locking of the crushed cast material to the binder is also good. These effects assist in improving the erosion resistance of a drill bit. The fatigue strength of the crushed cast material is thought to be lower than that of the macrocrystalline material as the crushing process induces small cracks in the material. In use of a drill bit, small cracks propagating through the binder to the tungsten carbide material may be able to propagate along and extend the cracks already present in the crushed cast tungsten carbide material. In drill bits manufactured using the macrocrystalline material, such cracks are not present in the tungsten carbide material and cracks forming within the binder must pass around rather than through the tungsten carbide material.
It is an object of the invention to provide a drill bit having an improved wear resistance compared to drill bits manufactured using the materials mentioned above.
According to a first aspect of the invention there is provided a drill bit of the matrix type having a bit body comprising a tungsten carbide material bound with a binder material, wherein the tungsten carbide material includes at least some tungsten carbide particles of generally spherical shape.
The, generally spherical tungsten carbide particles are preferably of a type having a relatively hard central core and an outer skin of relatively low hardness. The outer skin conveniently includes a high temperature form of tungsten carbide which is relatively ductile and is amenable to wetting by the binder material. The outer surface of the sphere is generally quite rough, providing a much greater surface area for bonding by the binder than the generally smooth surfaces of crushed and macrocrystalline tungsten carbide.
The use of particles of generally spherical form permits an increase in the density with which the particles can be packed in a mold during the manufacturing process. The use of particles of the type having a relatively hard central core and a relatively soft, ductile outer skin results in the drill bit being of good abrasion resistance (as the core is hard) and good fatigue strength.
According to another aspect of the invention there is provided a drill bit of the matrix type having a bit body comprising a tungsten carbide material bound by a binder material, wherein the tungsten carbide material comprises at least some particles having a relatively hard central core and a softer, relatively ductile outer skin.
The central core conveniently has a hardness of at least 2000HV100, the hardness preferably being approximately 2100HV100. The outer skin preferably has a hardness falling within the range 1250-1750HV100, and is conveniently approximately 1500HV100.
According to another aspect of the invention there is provided a drill bit of the matrix type having a bit body comprising a tungsten carbide material bound by a binder material, wherein the tungsten carbide material includes at least some particles which include a high temperature phase of tungsten carbide.